This invention relates to a carbothermic process for producing magnesium from magnesium oxide. More particularly, the present invention provides means for greatly decreasing the back-oxidation of magnesium vapor by carbon monoxide as the furnace gases are cooled, which has heretofore prevented successful commercial development of a carbothermic magnesium process.
It has long been recognized among those skilled in the art that production of magnesium from magnesium oxide by reduction with carbon in an arc furnace is theoretically the most efficient and cheapest method for the commercial production of magnesium. In particular, it offers many advantages over metallothermic processes where a reducing agent such as silicon or aluminum requires an expensive preliminary operation requiring electric energy. In a carbothermic process, the magnesia-containing raw material is subjected to high temperature reduction by carbon in an arc furnace. The product initially produced comprises a vaporous composition which is theoretically approximately a 50--50 mixture of magnesium vapor and carbon monoxide gas. Thermodynamic calculations for the reaction show that the theoretical reaction equilibrium temperature at atmospheric pressure is about 1875.degree. C. and in order to carry out the reaction at a reasonable rate, it is has been found necessary to operate at temperatures on the order of 2000.degree. C. This thermodynamic constraint is undesirable since it causes extreme difficulty in cooling the furnace gases down to the equilibrium temperature of about 1875.degree. C., rapidly enough to prevent excessive back oxidation of magnesium by reaction with carbon monoxide to form magnesium oxide. The reaction in question, which is reversible as shown,
Ti MgO+C.revreaction.Mg+CO
reaches equilibrium at about 1875.degree. C. at atmospheric pressure of the CO, proceeds violently toward oxidation of magnesium if the gaseous mixture of Mg and CO is cooled below that temperature and above 1875.degree. C. to the operating temperature. Above 2000.degree. C., the reaction proceeds rapidly to the right to effect substantially complete reduction of the magnesium oxide to form magnesium vapor and carbon monoxide.
On the other hand, I have found that at temperatures below about 1875.degree. C., the rate of back oxidation of magnesium vapor by carbon monoxide decreases rapidly with the temperature to such an extent that the reaction rate at 1500.degree. C. is less than 1% of the rate at the equilibrium temperature of 1875.degree. C. and at a temperature of 1100.degree. C. a little above the dew-point, substantially no back oxidation of magnesium occurs provided the magnesium vapor is condensed quickly.
Prior attempts to cool the reaction product from the carbothermic reaction comprising magnesium and carbon monoxide down to the temperature where back oxidation becomes insubstantial have proven unsuccessful. It has been proposed to utilize a wide variety of coolants including methane gas, solid powdered magnesium and a stream of relatively cool (650.degree. C.-670.degree. C.) liquid magnesium. While it has been proposed to utilize these coolants in amounts sufficient to extract the heat necessary to cool the vaporized magnesium down to the point where insubstantial back oxidation occurs, each approach has proven unsuccessful since the cooling obtained is too slow. That is, each of these approaches effects a rate of cooling such that the vaporized magnesium experiences a substantial residence time within the temperature range of between about 1875.degree. C. and 1500.degree. C. Since, during the residence time between 1875.degree. C. and 1500.degree. C. the vaporized magnesium is an intimate contact with carbon monoxide, substantial back oxidation occurs to an extent as to render these processes uneconomical and commercially unfeasible.
It would be highly desirable to provide a carbothermic process for forming magnesium from magnesium oxide wherein vaporized magnesium can be recovered without substantial back oxidation. Furthermore, it would be highly desirable to provide such a process wherein the residence time for vaporized magnesium product in contact with carbon monoxide within the temperature range of 1875.degree. C. and about 1500.degree. C. is minimized. Furthermore, it would be highly desirable to provide such a process wherein substantially pure magnesium is recovered directly from the vaporous product gas.